Conventionally, in the manufacture of metal-made element parts used in automobiles or the like, usually, parts are joined to each other by arc welding or the like. For example, in the case of joining a shaft body to a plate, the shaft body is fitted into a hole formed in the plate by fitting engagement, and the whole circumference or a portion of the fitting engagement portion in which the shaft body is fitted is welded by arc welding using filler metal. Further, a resistance welding method such as a spot welding method or a projection welding method, and a method of joining parts by caulking has been also used.
However, the joining method such as arc welding has the following drawback. That is, due to the thermal deformation of the plate, the shaft body or the like by welding heat generated in arc welding or the like and hence, a base material of the plate, the shaft body or the like thermally deteriorates, and a size of the plate, the shaft body or the like is distorted, and such thermal deterioration and the distortion in size inevitably affect the accuracy of size of the plate, the shaft body or the like. In this case, there arises a drawback that finishing after welding such as working for increasing accuracy of a product by applying post-working after welding or shaving of undesired filler metal at a welded portion takes considerable time, effort and cost.
A lap resistance welding method has been mainly used as a resistance welding method, and in all lap resistance welding methods, joining is performed by forming a molten structure referred to as nuggets at a joining portion. In the lap resistance welding method, the only way to strengthen the welding is to increase the number of nuggets thus eventually giving rise to a drawback that the thermal deterioration of a joining base material and the exerting of adverse influence on accuracy of size cannot be avoided. Although melt welding exhibits a large joining strength, the base material is thermally affected in a wide range and hence, melt welding causes thermal deterioration and considerably affects accuracy of size adversely thus giving rise to a drawback that post-working or the like is necessary thus pushing up a manufacturing cost.
In view of such drawbacks, an applicant of this application and others developed a press-fit joining method where an electric current is supplied between a plate body and a shaft body while press-fitting the shaft body into a hole portion of the plate body with a predetermined pressure thus generating electric resistance heat at a joining portion between the plate body and the shaft body so that the shaft body is press-fitted into the hole portion of the plate body whereby the hole portion of the plate body and the shaft body are joined to each other by solid-phase diffusion joining, and such a press-fit joining method has been put into practice (see Patent literatures 1, 2, for example).
FIG. 15 is a view for explaining a conventional press-fit joining method. In the conventional press-fit joining method, as shown in FIG. 15, an electric current is supplied between a plate body and a shaft body while press-fitting a shaft body 903 into a hole portion 902 of the plate body (plate) 901 with a predetermined pressure while having a press-fitting margin between the shaft body 903 and the hole portion 902 of the plate body 901 thus generating electric resistance heat at a joining portion between the plate body 901 and the shaft body 903, the shaft body 903 is press-fitted into the hole portion 902 of the plate body 901 whereby the hole portion 902 of the plate body 901 and the shaft body 903 are joined to each other by solid-phase diffusion joining. In FIG. 15, symbol 904 indicates a lower electrode, and symbol 905 indicates an upper electrode.
The conventional press-fit joining method is a method of joining a plate body and a shaft body by press-fitting which can manufacture products having excellent finishing accuracy and also excellent strength with a high economic effect.